The present invention relates to a CRT (cathode-ray tube) focus correcting method and CRT focus correcting circuit which are used for a display unit such as a display monitor and a television, and relates to a display unit provided with the CRT focus correcting circuit.
FIG. 1 is a block diagram showing an example of a configuration of a conventional CRT focus correcting circuit for correcting a CRT focus disclosed in Japanese Patent Application Laid-Open No. 4-357772(1992). This CRT focus correcting circuit is provided with a E.sup.2 P-ROM 1. The E.sup.2 P-ROM 1 stores horizontal correction data, which are synchronized with a horizontal synchronizing signal, and vertical correction data which are synchronized with a vertical synchronizing signal, and size data of a picture size according to these correction data. The horizontal correction data and vertical correction data are used for generating a parabola waveform voltage for correcting a CRT focus according to a picture size on a CRT screen.
In addition, the CRT focus correcting circuit is provided with a horizontal correction wave generating circuit 2 supplied with horizontal correction data from the E.sup.2 P-ROM 1 thereinto, a vertical correction wave generating circuit 3 supplied with vertical correction data from the E.sup.2 P-ROM 1 thereinto, and horizontal and vertical correction wave synthesis circuit 4 for synthesizing and amplifying respective outputs from the horizontal correction wave generating circuit 2 and vertical correction wave generating circuit 3 so as to output them to a CRT focus coil or a focus correcting electrode. The horizontal correction wave generating circuit 2 generates a parabola waveform voltage based on the supplied horizontal correction data, and the vertical correction wave generating circuit 3 generates a parabola waveform voltage based on the supplied vertical correction data.
The following describes an operation of the CRT focus correcting circuit having the above configuration.
The horizontal correction data and vertical correction data, which are used for generating parabola waveform voltages correcting a CRT focus according to a picture size on the CRT screen, as well as size data of a picture size according to those correction data are previously stored in the E.sup.2 P-ROM 1.
When information relating to a picture size is inputted from a picture size set circuit (not shown) composed of a variable resistor provided on the front surface of the display unit into the E.sup.2 P-ROM 1, the E.sup.2 P-ROM 1 reads out horizontal correction data and vertical correction data according to size data of the picture size with them being synchronized with a horizontal synchronizing signal and vertical synchronizing signal.
The read-out horizontal correction data and vertical correction data are given respectively to the horizontal correction wave generating circuit 2 and the vertical correction wave generating circuit 3, and the horizontal correction wave generating circuit 2 and vertical correction wave generating circuit 3 output voltage signals based on the respective correction data.
The voltage signals outputted respectively from the horizontal correction wave generating circuit 2 and vertical correction generating circuit 3 are given to the horizontal and vertical correction wave synthesis circuit 4 and are synthesized and amplified therein so as to be outputted to the CRT focus coil or the focus correcting electrode. As a result, the parabola waveform voltage according to a picture size on the CRT screen for correcting the CRT focus is applied to the CRT focus coil or the focus correcting electrode.
As mentioned above, in the conventional CRT focus correcting circuit, correction data are previously stored in a memory, the correction data are read out according to an external instruction in a picture size, and a parabola waveform voltage is generated based on the read-out correction data. For this reason, in the case that correction data which are not previously stored in the memory are required, a parabola waveform voltage which continuously changes (not discretely) cannot be obtained according to a changed picture size. Moreover, in order to store data of the parabola waveform voltage which continuously changes so that the data can comply with a change in a frequency of the horizontal synchronizing signal, it is necessary to store another correction data which fill up a gap between the conventional correction data and which are used for obtaining a parabola waveform voltage which discretely changes, thereby requiring a large memory capacity.